If you think your Apple watch is the coolest thing in timekeeping, think again. Scientists at Germany's Physikalisch-Technische Bundesanstalt (PTB) have just published a paper in the journal Physical Review Letters on the most accurate timekeeping device in history. It beats out the previous champs, atomic clocks, by a large margin. In fact, the new single-ion clock is so accurate that it may force us to redefine what a "second" is. To understand the single-ion clock, you have to understand its predecessor, the atomic clock. Scientists have spent a lot of time watching atoms. They've figured out that an atom's electrons do a lot of predictable and detectable things when they jump from energy level to energy level in an atom (remember those electron clouds you had to draw in school?). In an atomic clock, atoms are forced to sit relatively still and then blasted with energy from a known frequency (like lasers or microwaves). The electrons move as a result of this energy blast, and the clock counts those movements. The clock knows how much time has passed by combining two known things (the behaviors of the electrons and the properties of the energy blast) to figure out an unknown thing (how much time has passed). Accuracy in timekeeping is measured by how precisely the length of each 'tick' matches the length of every other 'tick'. The atomic clock's form of timekeeping is so accurate that our current definition of what a 'second' is comes from the best atomic clocks currently around — cesium atomic clocks. Scientists measure clock accuracy by their level of systematic uncertainty, which is a type of error rate. The Germans' single-ion clock kicks the atomic clock's timekeeping butt on that score. Its systematic uncertainty is 3 x 10-18. That's 100 times better than a cesium atomic clock, an accuracy that scientists have been trying to attain since physicist Hans Dehmelt said it was possible back in 1981. The PTB single-ion clock uses Yb+, an ion of ytterbium, but there are research projects in the U.K. and China on single-ion clocks based around other ions as well, such as Sr+, Ca+ and Al+. Dr. Christian Tamm, who works with the optical clocks at PTB, told us via email, "Each of these ion systems has particular advantages and disadvantages with respect to accuracy potential and technical complexity." Tamm adds, "Yb+ seems to have a high accuracy potential challenging Al+, but maybe a clock based on Yb+ is easier to realize and optimize." This discovery has way bigger implications than just making sure you're on time for your next swim meet. Super accurate timekeeping has a huge impact on GPS and even the Internet. PTB hopes to use its clock in order to test physical theories. An all-optical atomic clock, recently demonstrated by researchers at the US National Institute of Standards and Technology (NIST; Gaithersburg, MD), produces about 1 quadrillion "ticks" per second and promises to be as much as 1000 times more accurate than the world's current standard in time measurement-cesium-based microwave atomic clocks. "Our optical clock, as well as those being developed by other groups around the world, should give scientists an even finer-grained view of the physical world, much as precision spectroscopy in the past 50 years has opened the door to an improved understanding of many fundamental aspects of atoms and molecules," says NIST physicist Scott Diddams. For the past 50 years, microwave atomic clocks have set standards for precision time and frequency metrology. Today, the most precise clocks are based on a natural atomic resonance of the cesium atom—the atomic equivalent of a pendulum. For example, NIST-F1, one of the world's most accurate time standards based on microwave atomic clocks, neither gains nor loses a second in 20 million years. The new clock, however, is designed to neither gain nor lose a second in 3 billion years. The optical clock at NIST relies on a combination of advances in physics: the trapping and cooling of atoms and ions with lasers; frequency-stabilized lasers; and a new optical frequency "comb" that uses a femtosecond laser with nonlinear optical fibers to provide a simple, direct, and exact linkage between microwave and optical frequencies. The researchers built their optical clock with a stable continuous-wave (CW) laser oscillator that is frequency doubled and locked to a narrow ultraviolet (282-nm) transition of a single, trapped and laser-cooled mercury ion (Hg+). Once stabilized, the frequency of the laser light was coherently divided down to lower frequencies with a femtosecond mode-locked laser that ultimately produced an electronic output at a frequency of 1 GHz. The envelope of the pulse train was made synchronous with the optical phase of the CW laser with approximately 532,361 optical cycles between pulses, which provides the "clock ticks" that are coherently connected to the Hg+ transition. By comparison to a laser-cooled calcium optical standard, an upper limit for the fractional frequency instability of 7 x 10-15 was measured in 1 s of averaging—a value substantially better than that of the world's best microwave atomic clocks. A higher transition frequency can produce a more stable frequency standard. This is the primary advantage of an optical atomic clock over a microwave clock, as the operating frequency is 100,000 times higher—providing a finer division of time and thus potentially higher precision. "From a technological standpoint, there is little dispute that stable and accurate microwave atomic clocks have greatly improved navigation and communications," Diddams says. "It is likely that optical clocks of the future will have a similarly important impact. Of particular interest will be the continued application of optical frequency standards in spectroscopy and the improved determination of the fine structure constant and the Rydberg constant. As measurement accuracy improves, metrologists may find themselves in the unique position of being able to observe physical 'constants' evolve in time." REFERENCE
Whether you sport a smartwatch or a Rolex, you likely believe the time your watch of choice tells you. We’re here to break it to you though: These timepieces are grossly inaccurate compared to the most accurate clocks in the world. But there is no such thing as perfect, it seems — because this year scientists made the most accurate clocks even more accurate. INVERSE is counting down the 20 science discoveries that made us say “WTF” in 2021. This is #5. See the full list here. What is an atomic clock?Atoms exist at different frequencies. If you recall high school science classes, you might recall putting magnesium in a flame and it then emitting a very bright light — this is essentially a frequency. Atomic clocks use these frequencies — specifically, absorbing and emitting photons at regular intervals to keep time. They are the most accurate clock we have to measure time in seconds. A common kind of atomic clock uses a form of cesium called cesium-133. In this case, microwave energy interacts with atoms belonging to a form of cesium called cesium-133. The cesium-133 oscillates between two states and emits a pulse, over and over. Since 1967, the formal definition of a second has been: “The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.” In other words, in a single second, a cesium-133 atom transitions some 9,192,631,770 times. There is a tiny degree of inaccuracy here, however: There are minuscule differences in the time kept by the atoms, and attempts to quantify these differences had an accuracy of up to 17 digits. Such knowledge matters — if you are able to better account for the errors, then you can more accurately gauge the exact period of a second using an atomic clock. In other words, increasing the degree of accuracy here may mean redefining the second itself. What’s new — In a study earlier this year, researchers laid the groundwork to do exactly that: A research team based in Colorado decided to try to improve on atomic clocks’ design by testing whether transmitting data via a length of fiber-optic cable or via lasers. They wanted to know:
Together, the results suggest the new methodology could provide 18-digit accuracy to the measure of the difference between atoms. They published their findings in the journal Nature in March. How they did it — The team devised three atomic clocks that used other elements:
Charles H. Townes at Columbia University shows one of the first atomic clocks.Bettmann/Bettmann/Getty Images The aluminum-ion and ytterbium clocks were kept in a lab in Boulder, Colorado. The strontium clock was placed about a mile away in a different lab. Then, the researchers spent months sending data points back and forth between the clocks using a little more than two miles worth of fiber optic cabling or shooting data in the form of laser pulses across a free-space link between the labs (literally an optical light signal traveling freely across a distance). They found the free-space link no different in terms of certainty than the fiber-optic cables. Ultimately, this is the most accurate measurement made to date of the ratios between these clocks — and offers a new foundation for innovating atomic clocks as a whole. Why it matters — Back in March, we talked to Rachel Godun, a senior research scientist in the Time and Frequency group at the National Physical Laboratory in the U.K. She told Inverse the use of free-space links to connect clocks could prove a game-changer for people far beyond the study of physics: “The authors’ demonstration that high accuracy clocks can be connected by free-space links, without needing an optical-fiber infrastructure, is exciting because it opens up possibilities for applications outside the laboratory, such as land surveying.” The researchers’ work could also, one day, entirely redefine what we think “a second” actually means. What’s next — Atomic clocks are incredibly accurate, but the technology they use is also old by today’s standards. The point of this study wasn’t so much to pick out a new and improved atomic clock, but rather further refine how these elements’ time-keeping accuracy was compared. Once those standards are in place, it opens the door to new clocks and more innovation — and a new definition of the second. If we manage that, then it could help physicists further test fundamental theories of the universe, like relativity and dark matter, by clocking atomic movements with greater precision. INVERSE is counting down the 20 science discoveries that made us say “WTF” in 2021. This is #5. Read the original story here. |
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